Solid state chemical micro-reservoirs

ABSTRACT

The present invention relates to a method for storing chemical reagents in micro-reservoirs, a chemical sensor system and a chemical micro-reactor.

This is a division of application Ser. No. 07/834,199, filed on Feb. 12,1992, now U.S. Pat. No. 5,262,127.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of storing chemical reagents.

2. Discussion of the Background

Chemical microsensors are playing an increasingly important role in manyapplications where monitoring of chemical concentrations is required.Chemical microsensors have an advantage over conventional analyticaltechnology in terms of significantly lower cost, lower powerconsumption, lower weight, and faster response. Chemical microsensorsoffer the opportunity for remote sensing, distributed real-time sensingat multiple locations, and applications requiring portable analyticalinstrumentation. The miniaturization of the sensor surface makes itpossible to analyze extremely small amounts of sample. Typical chemicaltransducers encountered in chemical microsensing technology consist ofthin films which undergo a rapid change in physical properties wheninteracting with molecules of interest.

Chemical microsensors suffer the drawbacks of deactivation, limitedselectivity and limited sensitivity. The limited number of analyte/filminteractions dictates both the selectivity and sensitivity for chemicalmicrosensors. If it were possible to co-administer a co-reagent, boththe selectivity and sensitivity of existing thin film sensors could beimproved.

Chemical microsensor lifetime is also a problem. The exposure of thefilm to an analyte sometimes produces irreversible binding and"poisoning" of the surface. Chemical microsensor lifetimes could beextended by regeneration of the film surface through exposure to areactivating chemical. It would be advantageous to both the selectivityand regeneration processes to be able to inject precise amounts ofchemicals near the thin film surface.

Further, chemical microsensors need to be calibrated for quantitativeanalysis. A known amount of analyte must be introduced to measure theresponse. Some sensors are very sensitive and must be recalibratedfrequently. There is presently no means for delivering micro amounts ofchemical agents to a chemical sensor.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a novelprocess for storing chemical reagents.

In another embodiment, the present invention relates to a chemicalmicrosensor containing a micro-reservoir for storing chemical agents.

Another embodiment, of the present invention provides for amicro-chemical reaction system containing a plurality ofmicro-reservoirs.

The object of the present invention is provided for by an active thinfilm deposited onto at least one "window". A selected chemical is thencharged onto the surface, at a certain temperature and pressure. Therelease of a chemical adsorbed onto or absorbed into the thin films canbe triggered by rapidly increasing the temperature of the active film.In addition, these windows are equipped with electrodes for monitoringin situ the progress and end point of chemical release. The electrodesare able to monitor the electrical characteristics of the active film asit is being charged and as the chemical is being released.

BRIEF DESCRIPTION OF THE FIGURES

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 provides a top view of the dielectric window used to practice thepresent invention.

FIG. 2 provides a cross sectional view of the dielectric window used topractice the present invention.

FIG. 3 provides a block diagram of a device containing multipledielectric windows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, a window element is coated with anactive solid film. The solid films, when charged or filled underappropriate conditions adsorb or absorb significant amounts of chemicalagents, forming compounds, complexes or solutions. After charging anelement at a certain temperature and pressure (for example, withhydrogen, oxygen or any other suitable reagent), the reagents can bestored for long periods of time at room temperature by the activeelement. Controlled release of the desired reagent is accomplished byrapidly increasing the temperature of the active film. The stored agentis then released in a controlled fashion.

The released reagents can be used as a calibration agent for chemicalsensors, for reactivation or regeneration of the deactivated or poisonedsensor elements, or as reagents for enhancing the sensitivity orselectivity of sensing elements. The transport of these reagents fromthe storage element to the target element can be accomplished andcontrolled using a variety of micro-fabricated and conventional methods.

The use of stored reagents for reactivation or regeneration ofdeactivated or poisoned sensor elements is of substantial interest forincreasing the operational lifetime and stability of a variety ofchemical sensors. Stored reagents may enhance the sensitivity orselectivity of sensing elements for gases which are now difficult todetect.

Chemical micro-storage devices may also find application inmicro-reactors systems for well-controlled production of small amountsof high-value added specialty chemicals, where accurate dosing of minuteamounts of a given reactant is required. The stored reagents may be usedto perform localized chemical reactions with great precision andreproducibility. Chemical microstorage devices may also findapplications in micro-reactor systems for synthesis of small amounts ofreactive chemicals which are difficult to store for extended periods.

The advantages of storing micro-reservoirs of chemical reagents isrealized through a chemical sensor system containing a conventionalchemical sensor and a micro-reservoir. The micro-reservoir contains achemical reagent to be used in conjunction with the chemical sensor forsuch things as calibration, reactivation or selectivity/sensitivityenhancement. As a chemical sensor, conventional sensors may be used suchas those described in U.S. Pat. No. 4,953,387.

Active films for micro-storage elements span the periodic table,suitable films are any film which is capable of reversibly absorbing achemical compound. Suitable films include but are not limited to metals,metal oxides insulators, semiconductors, metal alloys, inorganic solids(i.e. hydrides, oxides, nitrides, carbides, sulfides, halides, sulfates,carbonates, intermetallic compounds and solid solutions thereof),intercalation compounds, zeolites, pillared clays, organic solids,organometallic compounds, oligomers and polymers.

The active films of the present invention include but are not limited togold, platinum, gold-platinum, titania, titanium carbide, platinum-tin,tin oxide, titanium dioxide, zinc oxide, zirconium oxide, palladium,ruthenium, nickel, copper, rhodium, molybdenum, iridium, iron, cobalt,titanium, vanadium, tantalum, tungsten, chromium, manganese, aluminum,tin, magnesium, osmium, zinc, silver or combinations thereof.

The thickness of the thin film is from 0.5 nm to 25 nm, preferably 5 nmto 10 nm.

As chemical reagent, suitable gases include hydrogen, oxygen, CO₂, CO,NO_(x), EtOH, MeOH, iPrOH, amines (such as ammonia, methylamine,ethylamine), chlorine, bromine, fluorine and iodine andchloro-aromatics. Any chemical which may be reversibly bound to achemical film then released at elevated temperature is suitable topractice the present invention.

The windows have electrodes for monitoring in situ the progress andend-point of the charging and discharging of the micro-reservoirs. Thepresent windows are equipped with from two to four electrodes.Additional electrodes are also possible. A four-point-probe electrodeconfiguration is capable of monitoring window impedance, which changesas a function of the absorption or release of a chemical.

The process of the present invention is practiced by exposing the arrayof "windows" coated with an active film to a chemical agent. Thechemical agent is adsorbed onto the surface or into the bulk of theactive film where it can be stored at room temperature for prolongedperiods of time.

The amount of chemical reagent stored on or in a micro-reservoir mayvary depending on the reagent, active film and the surface area of thefilm, anywhere from 10⁻⁵ -10⁻³ mmol may be stored.

A suitable dielectric window for practicing the present process isdescribed in U.S. Pat. No. 4,953,387. The dielectric window consists ofa thin dielectric window supported by an outer rim of thicker silicon.The window is composed of successive layers of silicon dioxide, siliconnitride, and silicon dioxide which are stress compensated so as to be inmild tension. In a preferred embodiment the window forms a 1.3 mm-squareby 1.3 μm-thick window. The window is successfully micromachined from awafer bulk.

A meandering boron diffused silicon heater is located under the windowwhich allows for heating temperatures above 1200° C. in a fewmilliseconds. The dielectric window provides a high degree of thermalisolation from the chip rim, ensuring high heating efficiency andlow-power operation. The structure also allows for the surface of thewindow to be planar on its upper surface. Additional structures can beintegrated onto such windows including thermocouples for independenttemperature measurements, contact resistant measurement structures anddifferent film geometries to evaluate effects of surface area onsensitivity. A rim 14 of thick silicon provides structural stability tothe device.

The window is fabricated beginning with a 1175° C. solid-source borondiffusion into a patterned wafer to define the rim area of the siliconchip from the front and the pattern for defining the final window andchip size from the back. A thermal oxide is now grown, followed bypolysilicon deposition, doping and patterning for the thermacouples andany other device structures. Films of low pressure chemical vapordeposition Si₃ N₄ and SiO₂ complete the dielectric window stack. Abuffered HF etch and an rf plasma etch are used to open contacts throughthe SiO₂ and Si₃ N₄ to the buried heater and the thermal couple.Referring to FIGS. 1 and 2, the window features a thin-diaphragmdielectric window (11) in the center of the chip. A heater means (15)below this window allows for rapid temperature cycling of the device, inparticular of the dielectric window. Heater contacts (21) are providedto the heater means (15). The substrate (24) may be advantageouslydesigned with support means (20). The active film (12) is deposited ontothe dielectric window (11) on the face opposite the heater means (15).Electrodes (13) may be provided to monitor the progress of gasabsorption and release.

The substrate (24) and the dielectric window (11) of the present devicecomprises a SiO₂ /Si₃ N₄ /SiO₂ dielectric sandwich in mild tension. Thethickness of these measured as the ratio of the total thickness ofsilicon oxide layers to the thickness of the silicon nitride layer isgreater than 1:1, preferably greater than 2.5:1, and up to 6:1.

The active film is located on the surface of the dielectric window,opposite to the heater. The active film may be deposited by a wide rangeof techniques for depositing films, including but not limited tosputtering, chemical vapor deposition, low pressure chemical vapordeposition-evaporation, ion beam deposition and molecular beam epitaxy.Films deposited by sputtering and chemical vapor deposition arepreferred.

The present invention allows for the storage of multiple chemical agentsonto different "windows" each being able to be liberated in a controlledmanner. The basic device has two windows and can be abutted in a hybridset of four chips to obtain an 8-element window array in a singlepackage. The impedance shifts which occur when a chemical reagent isabsorbed are used as indicators of chemical storage. Each windowinterfaces with a host node via an 8-line front end standard interface.As a result of employing this standard, each chip requires only 10shared pads (including 3 for power and ground). Each window operatesindependently of each other and all can be monitored simultaneously. Thecircuitry designed for this device allows the temperature of each windowto be set from ambient to 1200° C. with 8 bits of resolution. In orderto accommodate a broad range of thin films, the on-chip inductancedetector can operate on films from 3 Ω to as high as 300 KΩ. Moreover,to allow the alternating current characterization of the deposited thinfilms, an on-chip pulse generator has been incorporated which offers 8bits of resolution over a frequency range of from direct current to 5MHz.

When multiple chemical reagents are stored on different windows, achemical micro-reactor is possible. By simultaneously liberating two ormore chemical reagents, a controlled chemical reaction can be conducted.Such a process is useful for preparing molecules which are difficult tostore since the desired molecule can be prepared as needed.

A block diagram of the active chip is shown in FIG. 4. The on-chipreadout electronics can be divided into three major units:interface/control, heater control and sensing module. Theinterface/control unit is responsible for communicating with the sensornode (via the front-end standard), and for receiving, interpreting, andexecuting commands sent by the node. It also generates proper controlsignals for the other units. The heater control maintains a stablewindow temperature which can be programmed by the user via thefront-end-standard interface. It also provides a voltage outputcorresponding to the window temperature which can be used tocharacterize the chip. Providing a stable temperature is challengingsince the energy required for the heater is a function of the gaspressure and its associated thermal conductivity.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for storing a chemical in a chemicalmicro-reservoir comprising:i) absorbing up to 10⁻³ mmol of apredetermined chemical onto an active film of a chemicalmicro-reservoir; ii) storing said predetermined chemical on said activefilm; and iii) heating said film to release said predetermined chemical,wherein said chemical micro-reservoir comprises: i) a substratecomprisinga) two opposed major surfaces including a dielectric windowregion; b) heater means situated on said dielectric window region on oneside of side opposed surfaces; and c) a conductivity cell electrodemeans situated on said dielectric window on the opposite surface fromsaid heater means; and ii) an active film deposited on said surfacecontaining said conductivity cell electrode means.
 2. The method ofclaim 1, wherein said active film has a thickness of from 0.5 nm to 25nm.